1. Field of the Invention
The present invention relates to a Complementary Metal Oxide Semiconductor (CMOS) image sensor and a method for manufacturing the same.
2. Description of the Related Art
In general, a Complementary Metal Oxide Semiconductor (CMOS) image sensor includes a photodiode and a CMOS device. The photodiode is a light receiving element. The CMOS device provides an electric signal using electric charge accumulated in the photodiode.
FIG. 1 discloses a prior art CMOS image sensor. The CMOS image sensor of FIG. 1 includes a P-type semiconductor substrate 1101, a field oxide film 1102, an N-type doping region 1103, a P-type high concentration doping region 1104, a first P-type well region 1105, and a second P-type well region 1106. The field oxide film 1102 is formed in a predetermined region of the P-type semiconductor substrate 1101. The P-type semiconductor substrate 1101 also defines a field region and an active region. The N-type doping region 1103 of photodiode source is formed to have a first depth at a whole surface or part of the active region of the P-type semiconductor substrate 1101. The P-type high concentration doping region 1104 is formed to have a second depth that is less than the first depth at a whole surface of the active region of the P-type semiconductor substrate 1101. The first P-type well region 1105 is formed to isolate the field oxide film 1102 from the N-type doping region 1103 of the photodiode source. The second P-type well region 1106 is formed in a different part of the active region. A drain is formed in the second P-type well region 1106. A gate is formed over the semiconductor substrate 1101 with a gate insulating film being interposed therebetween.
FIG. 2 discloses mask patterns for forming the prior art CMOS image sensor of FIG. 1. FIG. 2 includes a mask pattern 1201, an active region mask pattern 1202, and a mask pattern 1203. When forming the prior art CMOS image sensor of FIG. 1, the mask pattern 1201 can be used to form the N-type doping region 1103, the active region mask pattern 1202 can be used to define the field region and the active region, and the mask pattern 1203 can be used to form the first P-type well region 1105. The first P-type well region 1105 is formed to isolate the field oxide film 1102 from the N-type doping region 1103 of the photodiode source and from the second P-type well region 1106.
FIG. 3 discloses a doping concentration of impurity ions in a depth direction of the prior art CMOS image sensor of FIG. 1. The log scale of FIG. 3 discloses the doping concentration of impurity ions in a vertical direction of a center of the N-type doping region 1103 of the prior art CMOS image sensor of FIG. 1. FIG. 3 discloses that the N-type doping region 1103 is constructed to have a doping concentration lower than that of the P-type semiconductor substrate 1101.
FIG. 3 also discloses a depletion region between two dotted lines. If electrons and holes generated by incident light are generated in the depletion region, the electrons and holes are not recombined and are isolated in respective different directions by an electric field formed within the depletion region. In this case, the electrons can be accumulated in the N-type doping region 1103. Therefore, the depletion region should have a large volume in the active region to effectively capture incident light at a low illuminance. However, if the depletion region increases as disclosed in FIG. 3, this results in a corresponding reduction in capacitance. Thus, the maximum amount of charge capable of being captured under any condition of variation amount of voltage given for the N-type doping region 1103 decreases.
FIG. 4 discloses a dynamic range of the prior art CMOS image sensor of FIG. 1.dQ=Cpd*dV  [Equation 1]where
dQ: variation of amount of charges depending on variation of voltage,
Cpd: capacitance of N-type doping region 1103, and
dV: variation of voltage of N-type doping region 1103.
As disclosed in Equation 1, when a voltage (Vpd=Vp) applied to the N-type doing region 1103 is equal to zero after light is incident and electrons are accumulated, a maximum capacity of the N-type doping region 1103 for accumulating electrons generated by light is equal to Cpd*Vp. This characteristic is expressed using a dynamic range as shown in FIG. 4.
One difficulty associated with the photodiode structure of the prior art CMOS image sensor of FIG. 1 is the trade off between sensitivity and dynamic range. In particular, an expansion in a vertical direction of the depletion region in the prior art CMOS image sensor of FIG. 1 leads to an improvement of sensitivity at a low illuminance with the improvement of photoelectric conversion efficiency. However, the expansion of the depletion region results in a reduction of a capacitance of the photodiode. Thus, a reduction of a capacity of a pixel device results in a deterioration of a dynamic range.